Switching DC-DC voltage converters are commonly used in electronic systems. Variations of DC-DC voltage converters include: buck converters (the output voltage is less than the input voltage), boost converters (the output voltage is greater than the input voltage) and buck-boost converters (the output voltage may be less than, equal to, or greater than the input voltage). Other variations include inverting converters (the output voltage has an inverse polarity relative to the input voltage) and non-inverting converters (the output voltage has the same polarity as the input voltage).
FIG. 1 illustrates an example of a prior-art, four-switch, non-inverting, buck-boost DC-DC voltage converter 100. The converter 100 has an inductor L and four switches (SW1, SW2, SW3, SW4). The switches (SW1, SW2, SW3, SW4) are typically implemented by field-effect transistors. A controller 102 monitors the input and output voltages (VIN, VOUT) and controls the switches (SW1, SW2, SW3, SW4) in the converter 100. In controller 102, a closed-loop system (not illustrated) adjusts the switching duty cycle based on the difference between VOUT and a reference voltage VREF. Typically, the switching cycle frequency is constant except for light loads (as discussed in more detail below).
For reasons that will be discussed in more detail below, the controller 102 typically operates the converter 100 in one of three modes: buck mode (the input voltage is at least 10% greater than the output voltage), buck-boost mode (the output voltage is within +1-10% of the input voltage), and boost mode (the output voltage is greater than 10% above the input voltage).
FIG. 2A illustrates the sequence of switch states when the converter 100 is operating in buck mode. In the buck mode, switch SW2 is always OFF, switch SW4 is always ON, and switches SW1 and SW3 alternate (when one is ON the other is OFF).
FIG. 2B illustrates the sequence of switch states when the converter is operating in boost mode. In the boost mode, switch SW1 is always ON, switch SW3 is always OFF, and switches SW2 and SW4 alternate.
FIG. 2C illustrates the sequence of switch states when the converter 100 is operating in buck-boost mode. In the buck-boost mode, all four switches (SW1, SW2, SW3, SW4) are active, with switches SW1 and SW2 always ON or OFF together, with switches SW3 and SW4 always ON or OFF together, and switch pairs (SW1, SW2) and (SW3, SW4) alternate.
In one of the two switch states in buck mode (FIG. 2A) and in boost mode (FIG. 2B), the voltage across the inductor L is VOUT−VIN. In the buck-boost mode, VOUT−VIN can become very small and the current in the inductor L will change very little during the time that the voltage across the inductor is VOUT−VIN is small. Therefore, in the buck-boost mode (FIG. 2C), the voltage across the inductor is either VIN or VOUT to ensure adequate energy storage in the inductor. However, the switching sequence of FIG. 2C is relatively inefficient due to energy losses in switching four switches. Accordingly, in buck mode and boost mode, the converter 100 operates in a relatively efficient two-switch sequence enabled by the magnitude of VOUT−VIN.
Another possible mode is light-load (low load current). In light-load conditions the inductor current can fall to zero or even reverse. In general, current reversal results in wasted energy. For efficiency, a controller 102 may include circuitry to detect inductor current reversal and switch to a separate discontinuous conduction mode (DCM). This is also known as a pulse skipping mode. During DCM, the operating frequency of the converter is typically proportional to the load current. Typically, improving efficiency during light-load conditions increases controller complexity and may impact load transient response time.
The controller 102 needs to change modes as the input voltage varies, needs to change modes as the load varies, and needs to respond rapidly to a load transient. The number of possible mode changes and mode change combinations make buck-boost controllers complex, and there are typically trade-offs between controller complexity and efficiency. There is a need for simplification of buck-boost controllers.